Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 3.3
Structure of (a) SWCNTs and (b) MWCNTs. (Reproduced with permission from Wang, J.,
Electroanalysis
, 17, 7-14,
2005.)
Sketches of aligned and random CNTs are shown in Figure 3.3b. Usually, CNTs were syn-
thesized by the following three techniques, and they are (1) the carbon arc-discharge tech-
nique [41-45], (2) the laser-ablation technique [46-48], and (3) the CVD technique [49-51]. Of
these, the arc-discharge technique results in high quality of single-walled CNTs (SWCNTs)
and multiwalled CNTs (MWCNTs). Furthermore, during the growth process, MWCNTs
do not require a catalyst, whereas the SWCNTs require a catalyst for its growth. Although
CNTs look promising, they are also plagued with certain limitations, which prohibit their
use on a large scale.
Fullerenes
Fullerenes, since their discovery in 1985, have captured the imagination of scientists
because of their unique physical and chemical properties. Buckminsterfullerene (C
60
) is
considered a truncated icosahedron containing 60 carbon atoms with C5-C5 single bonds
forming pentagons and C5-C6 double bonds forming hexagons [52]. The diameter of a
C
60
fullerene molecule is 0.7 nm, and it is an important member of the nanomaterials fam-
ily. However, their poor solubility in aqueous solvents coupled with a tendency to form
aggregates in aqueous solutions makes it an unattractive candidate in biological applica-
tions [53-55]. To overcome this problem, researchers functionalized fullerenes by various
chemical and supramolecular approaches [56-58]. Some of the functionalized fullerenes
have excellent solubility in polar solvents and easily overcome the hurdles posed by C
60
.
Usually, fullerenes are synthesized by vaporizing graphite by resistive heating under care-
fully defined condition and isolating the product of the resulting soot by chromatographic
technique [59, 60]. On the other hand, fullerenes can also be obtained by the combustion
of simple hydrocarbons in fuel-rich flames [61]. Isolable quantities of fullerenes are also
resulted by chemical synthesis [62].
Graphene
Graphene is the name given to a two-dimensional crystalline form of carbon invented
by Geim and Novoselov in 2004 [63]. Graphene can be further envisaged as a flat mono-
layer of carbon atoms firmly packed into a honeycomb-like crystalline lattice in a two-
dimensional fashion [63, 64]. Graphene is considered a basic building block for graphitic
materials of all other dimensionalities [65]. For instance, graphene can be wrapped up
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